CK-12

CK-12 Foundation publishes free, open educational resources including interactive textbooks, videos, and simulations for K-12. Founded in 2007, it's grown into a major source of openly licensed digital content that educators can use and remix to fit their needs. The library covers science, math, social studies, languages, and more.

Teachers choose CK-12 when they have budget constraints or want to customize materials to match their specific curriculum. It's flexible for mixed-resource environments where educators pull CK-12 content alongside other tools instead of adopting a single platform.